Plastic waste pyrolysis with liquid recycle

This application claims the priority benefit of U.S. Ser. No. 63/055,374, filed Jul. 23, 2020, and U.S. Ser. No. 63/111,998, filed Nov. 10, 2020, the disclosures of which are incorporated herein by reference.

Systems and methods are provided for pyrolysis of plastic waste by using a liquid recycle as a transport medium for the plastic waste.

Recycling of plastic waste is a subject of increasing importance. Conventionally, polyolefins in plastic waste are converted by various methods, such as pyrolysis or gasification, to produce energy. While this provides a pathway for using waste plastic a second time, ultimately methods for generation of energy from plastic waste also result in conversion of the plastic waste into CO. To make the process fully circular, so that the polymers can be recycled for return to the same usage, these pyrolysis and gasification products need to go through further pyrolysis or conversion processes to return them back to the light olefin monomer. The olefin monomers can then be repolymerized back to the polyolefin for use in the same service. Unfortunately, this process to make light olefins is high in energy usage, capital required, and produces relatively low yields of the light olefin monomers. It would be desirable to develop systems and methods that can allow for a circular recycle path for polyolefins with improved olefins yields.

U.S. Pat. No. 5,326,919 describes methods for monomer recovery from polymeric materials. The polymer is pyrolyzed by heating the polymer at a rate of 500° C./second in a flow-through reactor in the presence of a heat transfer material, such as sand. Cyclone separators are used for separation of fluid products from solids generated during the pyrolysis. However, the resulting vapor phase monomer product corresponds to a mixture of olefins, and therefore is not suitable for synthesis of new polymers.

U.S. Pat. No. 9,212,318 describes a catalyst system for pyrolysis of plastics to form olefins and aromatics. The catalyst system includes a combination of an FCC catalyst and a ZSM-5 catalyst.

U.S. Pat. No. 5,481,052 describes an example of using a screw feeder to introduce plastic pellets into a fluidized bed pyrolysis reactor.

U.S. Pat. No. 6,861,568 describes a method for performing radical-initiated pyrolysis on plastic waste dissolved in an oil medium. Although recycle was generally described, the examples in the description all involved mixing fresh plastic waste with another type of petroleum oil that was different from the liquids formed by decomposition of plastics.

Chinese Patent CN101230284 describes methods for coking of plastic waste. The plastic waste is pulverized to form small particles. The resulting particles are fluidized using a screw extrusion conveyor, followed by heating and extrusion to convert the plastic waste into a semi-fluid state. The heated and extruded plastic waste is then stored at a temperature of 290° C. to 320° C. to maintain the plastic in a liquid state. The liquid plastic waste is then pumped into the coker furnace.

U.S. Pat. No. 9,920,255 describes methods for depolymerization of plastic material. The methods include melting and degassing a plastic feed to form molten plastic. A liquid crude fraction is then added to the molten plastic to reduce the viscosity prior to introducing the mixture of molten plastic and liquid crude into the pyrolysis reactor.

In various aspects, a method for performing pyrolysis on a plastic feedstock is provided. The method includes mixing a feedstock comprising plastic particles having an average diameter of 3.0 cm or less with a recycled effluent fraction to form a slurry of plastic particles in recycled effluent. A weight ratio of the recycled effluent fraction to the plastic particles can be 1.0 or more. The mixing can be performed at a temperature of 150° C. or less. The method can further include introducing the slurry of plastic particles in recycled effluent into a fluidized bed pyrolysis reactor. The slurry of plastic particles in recycled effluent can be exposed to a temperature of 300° C. or more for a thermal residence time of 30 seconds or less prior to the introduction into the fluidized bed pyrolysis reactor. The method can further include pyrolyzing the slurry of plastic particles in recycled effluent in the fluidized bed pyrolysis reactor at a temperature of 500° C. to 900° C. to form a pyrolysis effluent. A fluidized bed in the fluidized bed pyrolysis reactor can include heat transfer particles. The pyrolysis conditions can include a single pass conversion of 15 wt % to 50 wt % relative to a conversion temperature of 370° C. or less. The method can further include separating at least a portion of the heat transfer particles from the pyrolysis effluent. Additionally, the method can include separating the pyrolysis effluent to form a bottoms fraction including the recycled effluent fraction and at least one additional fraction, the bottoms fraction having a T10 distillation point of the conversion temperature or less. Optionally, the weight ratio of the recycled effluent fraction to the plastic particles can be 1.0 to 5.0.

In various additional aspects, a method for performing pyrolysis on a plastic feedstock is provided. The method includes mixing a feedstock containing plastic particles having an average diameter of 3.0 cm or less with a recycled effluent fraction to form a solution of plastic in recycled effluent, a weight ratio of the recycled effluent fraction to the plastic being 0.2 or more. The method further includes pyrolyzing the solution in a pyrolysis reactor under pyrolysis conditions (including a temperature of 500° C. to 900° C.) to form a pyrolysis effluent. The pyrolysis conditions can include a single pass conversion of 40 wt % or less relative to a conversion temperature of 370° C. or less. Additionally, the method can include separating the pyrolysis effluent to form a bottoms fraction including the recycled effluent fraction and at least one additional fraction, the bottoms fraction having a T10 distillation point of the conversion temperature or less. Optionally, the mixing can include mixing the feedstock containing plastic particles and the recycled effluent at a temperature of 120° C. or more for a mixing residence time of 1.0 seconds to 600 seconds. Optionally, the mixing can include forming a mixture at a temperature of 150° C. or less, and heating the mixture to a temperature of 120° C. or more for a mixing residence time of 1.0 seconds to 600 seconds.

All numerical values within the detailed description and the claims herein are modified by “about” or “approximately” the indicated value, and take into account experimental error and variations that would be expected by a person having ordinary skill in the art.

In various aspects, systems and methods are provided for conversion of polymers (such as plastic waste) to olefins. The systems and methods can include a recycle loop so that a portion of the pyrolysis effluent can be combined with solid plastic feedstock. In some aspects, a slurry of plastic feedstock particles in the recycle liquid can be used as the input flow to the pyrolysis reactor. A slurry can be formed by combining a liquid recycle stream and plastic particles at a temperature below the melting temperature of the plastic particles and in a weight ratio of recycle liquid to plastic particles between roughly 1.0 to 5.0 (i.e., between 1:1 and 5.0:1). Having a weight ratio of recycle liquid to plastic particles of 1.0 or more can provide sufficient fluid in the slurry so that the slurry has a suitable kinematic viscosity for transport (i.e., suitable for pumping and/or flowing) within a reaction system. At weight ratios of recycle liquid to plastic of greater than 5.0, the plastic particles will have an increased tendency to form a solution rather than a slurry as the mixture of recycle fluid and plastic particles is heated on the way to the pyrolysis reactor.

In other aspects, recycle liquid can be mixed with the solid plastic feedstock particles at a sufficiently high temperature and/or in a sufficiently high ratio of recycle liquid to plastic to result in formation of a solution. It is noted that a solution can potentially be formed by any weight ratio of recycle liquid to plastic if the mixing is performed at a temperature above the melting temperature of the plastic particles. However, as a practical matter, solutions with a manageable viscosity for transport within a reaction system can be desirable. In some aspects, a solution with a manageable kinematic viscosity for transport within the reaction system can correspond to a solution with a weight ratio of recycle liquid to plastic particles of 0.2 or more, or 1.0 or more, or 2.0 or more, or 4.0 or more, or 5.0 or more, such as up to 15 or possibly still higher. At higher recycle ratios, such as a weight ratio of recycle liquid to plastic particles of 4.0 or more (or 5.0 or more), it may be possible to form a solution at temperatures below the melting temperature of the plastic particles. Additionally or alternately, a solution with a manageable kinematic viscosity for transport can have a kinematic viscosity at 100° C. of 1000 cSt or less, or 100 cSt or less, such as down to 0.1 cSt or possibly still lower. Optionally, the recycled effluent can correspond to heavier fraction or bottoms fraction of the pyrolysis effluent.

One of the difficulties with processing a polymer-based feedstock by pyrolysis, such as a plastic waste feedstock, is managing the input flow of the feedstock into the pyrolysis reactor. Various types of plastics, such as polyolefins, have melting points that are well below typical pyrolysis temperatures. As a result, when using conventional methods for introducing plastic into a pyrolysis reactor, the plastic feedstock can end up in a mixed state corresponding to some solid phase plastic and some (melted) liquid phase plastic. Having a mixed phase feed can present difficulties, as a feeding mechanism that is suitable for moving solid phase particles can often have limited effectiveness for moving liquid phase materials. Similarly, a feeding mechanism that is suitable for moving liquid phase material can often have difficulty with transport of solid particles.

In addition to difficulties with moving liquid phase and/or solid phase material, the use of solid phase or liquid phase (molten) plastic as a feedstock can pose challenges within a reaction environment. For example, although molten plastic is technically a liquid, the viscosity of molten plastic can be relatively high. Additionally, molten plastic can exhibit non-Newtonian flow properties. These less desirable flow characteristics can make it difficult to move and distribute molten plastic within a reactor that is designed to handle liquid feeds. With regard to movement, solid plastic particles tend to behave similarly to other types of particles. Unfortunately, this means that solid plastic particles have similar limitations to other types of particles. For example, use of solid plastic particles as a feed can limit the types of reactors that can handle the plastic feed.

Conventionally, operating a pyrolysis reactor with a substantial product recycle rate is not a preferred option. A substantial product recycle can correspond to operating a reactor with a recycle ratio of 1.0 or more, where the recycle ratio is defined as the wt % of recycled effluent relative to the wt % of fresh input feed. Operating a reactor while using a recycle to fresh feed weight ratio (i.e., a recycle ratio) of 1.0 or more (or 2.0 or more, or 4.0 or more, or 5.0 or more) means that the amount of feed that can be processed within a given reactor footprint is substantially reduced. This results in corresponding increases in both equipment costs and operating costs for processing a given volume of feed. It has been discovered, however, that performing plastic waste pyrolysis with substantial recycle can provide processing advantages that overcome the conventional increases in equipment and processing costs.

Some advantages can be related to the ability to reduce the severity of the pyrolysis process. For example, naphtha boiling range and/or distillate boiling range portions of the pyrolysis effluent can be separated out as a product stream, while any heavier portions of the effluent can be used as a recycle stream. In such aspects, the recycling of the pyrolysis effluent can allow heavier portions of the input flow to pyrolysis to be selectively exposed to the pyrolysis conditions multiple times while allowing portions of the effluent in a desired boiling range to be removed from the system. Because the heavier portions of the effluent are recycled, a desired amount of net conversion of the fresh plastic feed can be achieved even though the single pass conversion within the reactor is relatively low. The resulting product stream can either be used directly as a product, or the product stream can be used as a feed for subsequent processing, such as a feed for a steam cracking process.

Other advantages can be related to the type of pyrolysis that can be performed. In some aspects, plastic particles can be dissolved in a recycled effluent portion. Such a solution of plastic in recycled effluent can be formed by exposing a mixture of recycled effluent and plastic particles to a sufficient temperature for a sufficient amount of time. Generally, forming a solution can involve heating a mixture of recycled effluent and plastic to a temperature that is roughly equal to or greater than the melting point for the plastic particles. For high weight ratios of recycled effluent to plastic, such as a recycled effluent to plastic weight ratio of 4.0 or more (or 5.0 or more), the time for forming a solution can be relatively short, such as 10 seconds or less in a well-mixed environment. As a result, for high weight ratios of recycled effluent to plastic, heating the mixture to a temperature of 120° C. or more (or 150° C. or more) prior to entering the pyrolysis reactor can generally be sufficient to form a solution. At lower weight ratios of recycled effluent to plastic, such as a weight ratio of 2.0 to 4.0 (or 2.0 to 5.0), additional time may be needed to form a solution. In aspects where a solution is formed, the advantages of using a liquid feed in a reaction system can be achieved while avoiding the difficulties of high viscosity and/or non-Newtonian fluid behavior associated with molten plastic. In such aspects, using a sufficient amount of pyrolysis effluent as a recycle stream can allow plastic to be pyrolyzed in any convenient type of reactor that can perform pyrolysis on a liquid hydrocarbon feed. Additionally, conventional methods for transport and distribution of liquid hydrocarbon feeds can be used to deliver the feed to the reactor and/or distribute the feed within the reactor.

In aspects where a slurry of plastic particles is formed in recycled effluent by using a recycle ratio of 1.0 to 5.0, the type of pyrolysis reactor can be somewhat constrained due to the presence of the plastic particles in the slurry. Additionally, in order to avoid melting of the plastic and/or forming a solution, it can be desirable to reduce or minimize the amount of time the slurry is at a temperature of 120° C. or more, or 150° C. or more, prior to entering the pyrolysis reactor. However, concerns about transfer of heat into the plastic particles can be reduced or minimized. In aspects where the plastic is physically processed prior to introduction into the slurry, so that the particles have a sufficiently small particle size, the heat transfer properties of the slurry can be similar to the heat transfer properties of the liquid recycled effluent used to form the slurry. Additionally, the slurry can be transported and/or distributed within a reaction system using conventional methods for handling slurry flows.

Still another advantage of using the pyrolysis effluent as a recycle stream is that compatibility issues between the fresh plastic feed and the recycled solvent or slurry fluid can be reduced or minimized. For example, if the fresh plastic feed corresponds to primarily paraffinic polymers, the resulting pyrolysis effluent can also be primarily composed of paraffinic species. If the fresh plastic feed includes a substantial portion of polystyrene or other aromatics, the resulting pyrolysis effluent can tend to include at least a portion of such aromatics.

In addition to reducing or minimizing compatibility issues, it has been discovered that using a recycle fraction as the solvent or carrier fluid for a slurry can impact the desired conditions for the pyrolysis reaction. In particular, the conversion of both the fresh plastic waste and the recycled liquid products is relevant for determining how to achieve a desired level of total conversion for a plastic waste feedstock.

In this discussion, a reference to a “C” fraction, stream, portion, feed, or other quantity is defined as a fraction (or other quantity) where 50 wt % or more of the fraction corresponds to hydrocarbons having “x” number of carbons. When a range is specified, such as “C-C”, 50 wt % or more of the fraction corresponds to hydrocarbons having a number of carbons between “x” and “y”. A specification of “C” (or “C”) corresponds to a fraction where 50 wt % or more of the fraction corresponds to hydrocarbons having the specified number of carbons or more (or the specified number of carbons or less).

In this discussion, the naphtha boiling range is defined as 30° C. (roughly the boiling point of Calkanes) to 177° C. The distillate boiling range is defined as 177° C. to 350° C. The vacuum gas oil boiling range is defined as 350° C. to 565° C. The resid boiling range is defined as 565° C.+. A fraction that is referred to as corresponding to a boiling range is defined herein as a fraction where 80 wt % or more (or 90 wt % or more, such as up to 100 wt %) of the fraction boils within the specified boiling range. Thus, a naphtha boiling range fraction is a fraction where 80 wt % or more (or 90 wt % or more) of the fraction boils within the naphtha boiling range. A fraction corresponding to a naphtha plus distillate fraction can have 80 wt % or more (or 90 wt % or more) of compounds that boil between 30° C. and 350° C. A fraction corresponding to vacuum gas oil plus resid can include 80 wt % or more (or 90 wt % or more) of compounds with a boiling point of 350° C. or more.

Plastic Feedstock

In various aspects, a plastic feedstock for pyrolysis can include or consist essentially of one or more types of polymers, such as polymers corresponding to plastic waste. The systems and methods described herein can be suitable for processing plastic waste corresponding to a single type of olefinic polymer and/or plastic waste corresponding to a plurality of olefinic polymers. In aspects where the feedstock consists essentially of polymers, the feedstock can include one or more types of polymers as well as any additives, modifiers, packaging dyes, and/or other components typically added to a polymer during and/or after formulation. The feedstock can further include any components typically found in polymer waste.

In some aspects, the polymer feedstock can include at least one of polyethylene and polypropylene. The polyethylene can correspond to any convenient type of polyethylene, such as high density or low density versions of polyethylene. Similarly, any convenient type of polypropylene can be used. In addition to polyethylene and/or polypropylene, the plastic feedstock can optionally include one or more of polystyrene, polyvinylchloride, polyamide (e.g., nylon), polyethylene terephthalate, and ethylene vinyl acetate. Still other polyolefins can correspond to polymers (including co-polymers) of butadiene, isoprene, and isobutylene. In some aspects, the polyethylene and polypropylene can be present in the mixture as a co-polymer of ethylene and propylene. More generally, the polyolefins can include co-polymers of various olefins, such as ethylene, propylene, butenes, hexenes, and/or any other olefins suitable for polymerization.

In this discussion, unless otherwise specified, weights of polymers in a feedstock correspond to weights relative to the total polymer content in the feedstock. Any additives and/or modifiers and/or other components included in a formulated polymer are included in this weight. However, the weight percentages described herein exclude any solvents or carriers that might optionally be used to facilitate transport of the polymer into the initial pyrolysis stage.

In aspects where the plastic feedstock includes less than 100 wt % of polyethylene and/or polypropylene, the plastic feedstock can optionally include 0.01 wt % or more of other polymers, or 0.1 wt % or more of other polymers. For example, in some aspects the plastic feedstock can include 0.01 wt % to 35 wt % of polystyrene, or 0.1 wt % to 35 wt %, or 1.0 wt % to 35 wt %, or 0.01 wt % to 20 wt %, or 0.1 wt % to 20 wt %, or 1.0 wt % to 20 wt %, or 10 wt % to 35 wt %, or 5 wt % to 20 wt %.

In some aspects, the plastic feedstock can optionally include 0.01 wt % to 10 wt %, or 0.1 wt % to 10 wt %, or 0.01 wt % to 2.0 wt %, or 0.01 wt % to 1.0 wt % of polyvinyl chloride, polyvinylidene chloride, or a combination thereof, and/or 0.1 wt % to 1.0 wt % polyamide. Polyvinyl chloride is roughly 65% chlorine by weight. As a result, pyrolysis of polyvinyl chloride (and/or polyvinylidene chloride) can result in formation of substantial amounts of hydrochloric acid relative to the initial weight of the polyvinyl chloride. In limited amounts, the hydrochloric acid that results from pyrolysis of polyvinyl chloride and/or polyvinylidene chloride can be removed using guard beds prior the secondary cracking stage. Additionally or alternately, calcium oxide particles can be added to the heat transfer particles in the pyrolysis reactor. With regard to polyamide, pyrolysis results in formation of NO. Limiting the amount of NOcan simplify any downstream handling of the contaminants removed from the pyrolysis effluent.

In various aspects, the plastic waste can be prepared for introduction as a plastic feedstock for mixing with recycled effluent to form a slurry or solution. Depending on the nature of the plastic feedstock, this can include using one or more physical processes to convert the plastic feedstock into particles and/or to reduce the particle size of the plastic particles.

For plastic waste feedstock that is not initially in the form of particles, a first processing step can be a step to convert the plastic feedstock into particles and/or to reduce the particle size. This can be accomplished using any convenient type of physical processing, such as chopping, crushing, grinding, shredding or another type of physical conversion of plastic solids into particles. It is noted that it may be desirable to convert plastic into particles of a first average and/or median size, followed by additional physical processing to reduce the size of the particles.

Having a small particle size can facilitate solvation of the plastic particles and/or distribution of plastic particles within a slurry in a desirable time frame. Thus, physical processing can optionally be performed to reduce the median particle size of the plastic particles to 3.0 cm or less, or 2.5 cm or less, or 2.0 cm or less, or 1.0 cm or less, such as down to 0.01 cm or possibly still smaller. For determining a median particle size, the particle size is defined as the diameter of the smallest bounding sphere that contains the particle.

Formation of Slurry or Solution of Plastic Particles in Recycled Effluent

The plastic particles can then be mixed with recycled effluent. In some aspects, the recycled effluent can be mixed with plastic particles to form a slurry. A slurry can be formed, for example, for mixtures with a weight ratio of recycled effluent to plastic particles of 1.0 to 5.0, or 1.0 to 4.0. In aspects where a slurry is formed, the recycled effluent can be at a temperature between 20° C. and 150° C. during the mixing, or between 50° C. and 150° C., or between 20° C. and 120° C., or between 50° C. and 120° C., or between 20° C. and 100° C., or between 50° C. and 100° C. This can allow the recycled effluent to mix with the particles without causing substantial melting of the particles.

In aspects where a slurry is formed the mixture of recycled effluent and solvent can be allowed to mix prior to further heating for a sufficient period of time to produce a well-mixed slurry. In some aspects, this can correspond to no additional mixing beyond the mixing time required to travel in conduits from the mixing vessel to the pyrolysis reactor. In other aspects, the mixing time can correspond to 1.0 seconds to 120 seconds. After this mixing, the slurry of plastic particles in recycled effluent can be heated to a desired temperature prior to entering the pyrolysis reactor. It is noted that heating the slurry can potentially result in melting of the plastic particles. However, after formation of the slurry, the high viscosity of the melted plastic can tend to reduce or minimize spread of the melted plastic within the slurry. So long as the slurry spends less than a thermal residence time at temperatures above the melting point of the plastic particles, the amount of melting can be reduced or minimized so that the mixture behaves substantially like a slurry. In some aspects, the slurry can be heated prior to entering the pyrolysis reactor to a pre-heating temperature of 200° C. or more, or 250° C. or more, or 300° C. or more, such as up to 500° C. or possibly still higher. In some aspects, the thermal residence time for the slurry at or above the pre-heating temperature of 200° C. or more, or 250° C. or more, or 300° C. or more, can be limited to reduce or minimize the impact of melted plastic particles. In such aspects, the thermal residence time for the slurry at or above the pre-heating temperature can be 30 seconds or less, or 20 seconds or less, or 10 seconds or less, such as down to 0.1 seconds or possibly having no pre-heating to a temperature of 200° C. or more.

One indicator that a slurry has been formed can be the kinematic viscosity of the resulting mixture. The kinematic viscosity of a slurry is not dependent on the viscous properties of the plastic particles. Instead, the kinematic viscosity of a slurry is based on the viscosity of the recycled effluent and the volume fraction of the plastic particles in the slurry. Equation 1 shows the general formula for how the kinematic viscosity of a slurry at a given temperature varies with the concentration of particles in the slurry.

In Equation 1, μis the viscosity of the slurry. μis the kinematic viscosity of the solvent/fluid corresponding to the liquid phase of the slurry in the absence of slurry particles (in this case, the kinematic viscosity of the recycled effluent). φ is the volume fraction of particles in the slurry. “φ” is an empirical constant corresponding to a “maximum” volume fraction of particles in the slurry. “a” is another empirical constant corresponding to an intrinsic viscosity constant. As shown in Equation 1, the viscosity of a slurry increases exponentially as the volume fraction of particles in the slurry is increased.

For a solution of plastic in recycled effluent, a different type of behavior is observed as the concentration of plastic is varied. Equation 2 shows the variation of kinematic viscosity with plastic concentration for a solution of plastic in recycled effluent. (2) ln(μ)=x ln(μ)+(1−x)ln(μ)+k(1−x)x(ln(μ)−ln(μ))/2

In Equation 2, μis the viscosity of the solution. μis the kinematic viscosity of the recycled effluent. “μ” is the kinematic viscosity of the plastic. “x” is the weight fraction of plastic in the solution. “k” is an empirical constant corresponding to an interaction parameter. As shown in Equation 2, at low concentrations of plastic, the kinematic viscosity of the solution is similar to the kinematic viscosity of the recycled effluent. As the amount of plastic increases, the kinematic viscosity varies as a weighted average of the natural logs of the kinematic viscosities for the recycled effluent and plastic, with an interaction term that is a maximum for a solution containing 50 wt % plastic.

In other aspects, the recycled effluent and plastic particles can be mixed to form a solution. To form a solution, the recycled effluent and plastic can be mixed and/or maintained as a mixture for a sufficient period of time, referred to as a mixing residence time, to allow the plastic particles to dissolve in the recycled effluent. This additional mixing and/or maintaining during the mixing residence time can occur at any convenient location prior to entering the pyrolysis reactor. For example, in some aspects, at least part of the mixing residence time can correspond to time that the mixture is flowing in a conduit from the initial mixing vessel to the pyrolysis reactor. In other aspects, all of the mixing residence time can be spent in a mixing vessel prior to transporting the solution to the pyrolysis reactor. Preferably, during the mixing residence time, the mixture of recycled effluent and plastic can be at a temperature above the melting point of the plastic particles to facilitate forming the solution. For example, during the mixing residence time to form the solution, the mixture of recycled effluent and plastic can be at a temperature of 120° C. or more, or 150° C. or more, such as up to 500° C. or possibly still higher. Optionally, the solution can be formed by initially mixing the recycled effluent and plastic particles at a temperature of 20° C. to 150° C., or 20° C. to 120° C., followed by heating the solution and maintaining the solution for the mixing residence time at a temperature of 120° C. or more, or 150° C. or more. Depending on the aspect, the mixing residence time can be 1.0 seconds to 600 seconds, or 1.0 seconds to 120 seconds, or 1.0 second to 10 seconds, or 10 seconds to 600 seconds, or 10 seconds to 120 seconds. After this mixing, the solution of plastic in recycled effluent can optionally be further heated to a desired temperature prior to entering the pyrolysis reactor. In some aspects, the solution can be heated prior to entering the pyrolysis reactor to a temperature of 200° C. or more, or 250° C. or more, or 300° C. or more, such as up to 500° C. or possibly still higher.

Processing Conditions—Initial Pyrolysis Stage

In various aspects, the slurry of plastic particles or the solution of plastic in recycled effluent can be fed into one or more pyrolysis reactors. After forming the slurry or solution, the feedstock is heated to a temperature between 400° C.-900° C., or 500° C.-900° C., or 400° C.-700° C., or 550° C. to 700° C., or 400° C.-500° C., for a reaction time to perform pyrolysis. The temperature can depend in part on the desired products. In aspects where a portion of the pyrolysis effluent will be exposed to a second thermal cracking stage, lower temperatures can be used in order to increase the yield of liquid phase products. Additionally, the amount of recycle that is desired can influence the temperature and/or the residence time, as lower single pass conversion rates can be used with recycle to achieve higher total conversion values. In some aspects, the reaction time where the feedstock is maintained at or above 500° C. can be limited in order to reduce or minimize formation of coke. In some aspects, the reaction time can correspond to 0.1 seconds to 6.0 seconds, or 0.1 seconds to 5.0 seconds, or 0.1 seconds to 1.0 seconds, or 1.0 seconds to 6.0 seconds, or 1.0 seconds to 5.0 seconds. The pyrolyzed feedstock is cooled to below 500° C. at the end of the reaction time.

In some aspects, diluent steam can also be fed into the pyrolysis reactor. The steam also serves as a fluidizing gas. In aspects where additional diluent steam is added, the weight ratio of steam to plastic feedstock can be between 0.3:1 to 10:1.

In some aspects, the temperature and reaction time can be selected to provide a desired rate of single pass conversion. The amount of single pass conversion can be defined relative to a convenient conversion temperature. As an illustrative example, if the conversion temperature is 370° C., specifying an amount of single pass conversion of 40 wt % or more would correspond to converting 40 wt % of the 370° C. products in the input flow to pyrolysis into 370° C.− products in the pyrolysis effluent. In other words, of the material in the input flow (recycled effluent plus fresh feed) that has a boiling point of 370° C. or more, 40 wt % of this material is converted to 370° C.− products. This can be referred to as an amount of single pass conversion relative to 370° C.

In some aspects, if the goal is to convert the plastic waste sufficiently to form a feed with a specified T95 distillation point or a specified final boiling point, it can be convenient to select the pyrolysis conditions based on a conversion temperature that roughly corresponds to the desired T95 distillation point or desired final boiling point. For example, if it is desired to use pyrolysis to form a net product stream with a T95 distillation point (or final boiling point) of 370° C. or less, then it may be convenient to specify the pyrolysis conditions relative to a conversion temperature of 370° C. Depending on the aspect, the conversion temperature used for defining single pass conversion can be 370° C. (or less), or 300° C. (or less), or 250° C. (or less), or 200° C. (or less), such as down to 150° C. or possibly still lower. In some aspects, the conversion temperature can also roughly be used to define the recycled portion of the effluent. In such aspects, if conversion is measured relative to 370° C., then the 370° C.+ portion of the effluent from pyrolysis can be recycled. As another example, in such aspects, if the conversion is measured relative to 300° C., then the 300° C.+ portion of the effluent can be recycled. It is noted that such a selection is for convenience, as selecting a conversion temperature for characterizing the reaction that is related to the cut point for forming the recycle fraction can simplify the relationship between conversion in the reactor and the recycle rate.

In various aspects, the single pass conversion relative to a conversion temperature can be selected to provide single pass conversion relative to the conversion temperature of 50 vol % and/or 50 wt % or less. It is noted that plastic waste feeds can tend to have boiling points well above 400° C. Thus, for a conversion temperature of 370° C. or less, substantially the entire amount of the fresh feed will have a boiling point greater than the conversion temperature. In this type of situation, if the single pass conversion rate within a reactor is 50 wt % or vol %, and if the unconverted material is recycled, by definition the amount of recycled material will equal the amount of fresh feed. Thus, by having single pass conversion of 50 vol % or less (or 50 wt % or less), the ratio of recycled material to fresh feed can be 1.0 or more.

Depending on the aspect, the amount of conversion relative to the conversion temperature can be 50 wt % or less. More generally, the single pass conversion relative to the conversion temperature can be 10 wt % to 50 wt %, or 15 wt % to 50 wt %, or 20 wt % to 50 wt %, or 10 wt % to 40 wt %, or 15 wt % to 40 wt %, or 20 wt % to 40 wt %, or 10 wt % to 35 wt %, or 15 wt % to 35 wt %, or 15 wt % to 25 wt %, or 10 wt % to 25 wt %. In such aspects, the conversion temperature can be 370° C. or less, or 300° C. or less, or 250° C. or less, or 200° C. or less, such as down to 150° C. or possibly still lower.

In addition to single pass conversion, the net conversion of fresh feed can be specified. The net conversion of fresh feed corresponds to the amount of fresh feed that boils above the conversion temperature that is eventually converted to compounds that boil below the conversion temperature, regardless of how many passes a particular compound makes through the reactor. In various aspects, the net conversion of fresh feed can be 90 wt % or more, or 95 wt % or more, such as up to substantially complete net conversion (relative to the conversion temperature) of fresh feed that is exposed to the pyrolysis conditions.